A wide variety of micro-electromechanical devices (MEMS) are known, including accelerometers, DC relay and RF switches, optical cross connects and optical switches, microlenses, reflectors and beam splitters, filters, oscillators and antenna system components, variable capacitors and inductors, switched banks of filters, resonant comb-drives and resonant beams, and micromirror arrays for direct view and projection displays. There are a wide variety of methods for forming MEMS devices, including a) forming micromechanical structures monolithically on the same substrate as actuation or detection circuitry, b) forming the micromechanical structures on a separate substrate and transferring the formed structures to a circuit substrate, c) forming circuitry on one substrate and forming micromechanical elements on another substrate and bonding the substrates side by side or in a flip-chip type arrangement, or d) forming micromechanical structures without any circuitry. Regardless of the actual method used, at some point in the manufacturing process for making MEMS devices, a sacrificial layer is generally removed in order to release the micromechanical structure. The released structure is then able to be actively actuated or moved, such as pivoting or rotation of a micromirror for a projection display or optical switch, or movement during sensing, such as an accelerometer in an automobile airbag system.
In its most simple form, the invention is directed to etching a material where a first etch removes a portion of the material and fully or partially physically removes the material, and where a subsequent etch removes additional material and removes the material chemically but not physically. The material can be a semiconductor material such as silicon, and the areas removed can be of any dimensions such as an elongated trench, a well or other area limited in size, or even an entire area across a substrate. The result of the first and second etches can also result in an undercut such as for microfluidic channels or for a thermal sensor, or for simply removing material in an IC process.
In another embodiment, the invention is directed to releasing a micromechanical structure, comprising: providing a substrate; providing a sacrificial layer directly or indirectly on the substrate; providing one or more micromechanical structural layers on the sacrificial layer; performing a first etch to remove a portion of the sacrificial layer, the first etch comprising providing an etchant and energizing the etchant so as to allow the etchant to physically, or chemically and physically, remove the portion of the sacrificial layer; and performing a second etch to remove additional sacrificial material in the sacrificial layer, the second etch comprising providing a second ethant that chemically but not physically etches the additional sacrificial material.
Another embodiment of the method is for etching a material on or within a substrate, comprising: performing a first etch to remove a portion of the material, the first etch comprising providing an etchant and energizing the etchant so as to allow the etchant to physically, or chemically and physically, remove the portion of the material; and performing a second etch to remove additional silicon, the second etch comprising providing an etchant that chemically but not physically etches the additional material.
Also disclosed is an apparatus that comprises an etching chamber; connected to the etching chamber, a first source of etchant capable of etching a target material at least partially physically; and connected to the etching chamber, a second source of etchant different from the first source of etchant and capable of etching the target material chemically but not physically.